The jury is still out on whether humans wiped out the mammoths . But researchers have found evidence that the disappearance of the woolly giants probably helped to change the climate. If the beasts were indeed hunted to extinction, that means human-driven climate change could have started long ago, the researchers say.
Like modern-day elephants, mammoths were nature's tree pruners. Their diet included large amounts of leaves and branches from young trees, and they kept the temperate northern lands of North America, Europe, and Asia well trimmed and mostly free of forests. In particular, mammoths feasted in the grasslands that had sprung up in Beringia, the land bridge between Siberia and Alaska that now sits at the bottom of the Bering Sea. But then, starting around 15,000 years ago, mammoth populations in the region plummeted. At about the same time, a genus of birch trees called Betula, native to the northern grasslands, underwent a population explosion.
Earth system scientist Chris Doughty of the Carnegie Institution for Science in Stanford, California, and colleagues decided to find out whether the change in Betula proliferation was connected to the disappearance of the mammoths. They started by studying Betula pollen records compiled from soil cores taken in Siberia and Beringia. Next, the team examined mammoth fossil records to establish the timeline for their disappearance from the region. They also used studies of elephant-feeding habits to estimate the impact of the loss of the mammoths on the grasslands, and they applied climate models to compute the effect of the vegetation change on global temperatures.
The results, the researchers report in a paper to be published in an upcoming issue of Geophysical Research Letters, suggest that when the mammoths disappeared, the Betula trees expanded across Beringia, forming forests that replaced as much as one-quarter of the grassland. The trees' leaves, which are darker than grasses, absorbed more solar radiation, and their trunks and branches, which jutted above the snowpack, continued the effect even in winter. The researchers calculated that the mammoths' disappearance contributed at least 0.1˚C to the average warming of the world around 15,000 years ago. Within Beringia, the warming due to the loss of the mammoths was probably closer to 0.2˚C, the team concluded.
"There must have been a large impact on global ecology and vegetation when so many elephantlike creatures went extinct at the end of the [Ice Age]," Doughty says. So it's likely that this sequence of events became "one of many climate [influences] during a period of rapid climate change," he says.
Paleoecologist Felisa Smith of the University of New Mexico, Albuquerque, questions some of the data. For example, she says, much of the dramatic increase in birch pollen occurred at least a thousand years before the mammoths went extinct. Still, she says, the study is intriguing, and it "adds to other recent findings suggesting that the human impact on the climate might have begun much earlier than previously suspected. [For decades], we've argued about what caused the extinction of [the mammoths] but have failed to move beyond this to ask about the consequences of the loss of these animals."